This invention relates to the field of raster distortion correction systems that are adaptable to a plurality of horizontal scanning frequencies.
Raster distortion correction, for example convergence and pincushion correction, is an important aspect of television performance, particularly for large direct view and projection television receivers and monitors. Projection television receivers can present a very difficult challenge to distortion correction systems due to the off-axis orientation of two of the three projection tubes and due to the need to provide separate distortion control systems for each projection tube. In projection television receivers, the green tube is usually in a central orientation. After the raster of the green tube is corrected, the rasters generated by the red and blue tubes must be corrected and converged to match the raster generated by the green tube.
Generally, distortion correction data can be stored in a digital memory, read out of memory, processed by an interpolator to provide additional correction data, converted to analog form, analog low-pass filtered and amplified for use as a convergence correction deflection signal. Each distortion correction circuit must be optimized not only for each projection tube""s internal geometry and mounting orientation, as well as the screen size and screen orientation, but for the horizontal scanning frequency of the input video signal as well. The analog low-pass filter, which can form an input for a preamplifier, is a part of the circuit that can be most sensitive to differences in horizontal scanning frequency. Moreover, many receivers presenting significant distortion correction challenges are already adapted to operate at the standard horizontal scanning frequency (1 fH) and twice the standard horizontal scanning frequency (2 fH). In fact, such receivers will also need to process video signals having a horizontal scanning frequency three times (3 fH) the standard frequency.
In the case of a projection television receiver, for example, it is necessary to provide each tube with a respective distortion correction circuit appropriate for each of horizontal and vertical deflection and optimized for each horizontal scanning frequency. Such optimization requires two unique low-pass filters for each tube, for each horizontal scanning frequency. If a projection television receiver were adapted to operate at 1 fH and 2 fH, then twelve independently optimized distortion correction circuits would be required for the tubes. If a projection television receiver were adapted to operate at 1 fH, 2 fH and 3 fH, then eighteen independently optimized distortion correction circuits would be required for the tubes. Even if most of the constituent parts of each distortion correction circuit could be standardized, it would still be necessary to design and manufacture many different low-pass filters. Moreover, each tube would also require a switching circuit for automatically selecting the appropriate filter for a particular tube and deflection direction (e.g., red vertical) and a horizontal scanning frequency.
It is therefore desirable to avoid the use of costly switchable analog filters in distortion correction systems. Distortion correction system that did not require sets of switchable analog low-pass filters would be advantageous for many reasons, including simpler operation, enhanced reliability, greater flexibility in adjustment and setup, reduced manufacturing costs, reduced design costs and reduced design time.
A distortion correction system in accordance with inventive arrangements comprises: means for storing distortion correction values; a switchable digital filter for processing the distortion correction values; the digital filter being switched on and supplying the distortion correction values together with interpolated distortion correction values as an output in a first operating mode; the digital filter being switched off in a second operating mode, only the distortion correction values being supplied as the output; a digital to analog converter for converting the supplied output to an analog distortion correction signal; an analog low pass filter coupled for receiving the analog distortion correction signal in each of the operating modes and generating an analog deflection signal; means for determining a horizontal scanning frequency of an input video signal; and, control means responsive to the determining means for selecting one of the operating modes for different horizontal scanning rates.
A further distortion correction system in accordance with inventive arrangements comprises: means for storing distortion correction values; a digital filter for processing the distortion correction values; the digital filter supplying the distortion correction values together with a first number of interpolated distortion correction values in a first operating mode; the digital filter supplying the distortion correction values together with a second number of interpolated distortion correction values in a second operating mode; a digital to analog converter for converting the supplied distortion correction values to an analog distortion correction signal; an analog low pass filter coupled for receiving the analog distortion correction signal in each of the operating modes and generating an analog deflection signal; means for determining a horizontal scanning frequency of an input video signal; and, control means responsive to the determining means for selecting one of the operating modes for different horizontal scanning rates.
The digital filter can be advantageously switched off in a third operating mode such that only the distortion correction values is supplied as an output to the digital to analog converter. In this case, the determining means advantageously selects the first operating mode when a first horizontal scanning frequency is identified, selects the second operating mode when a second horizontal scanning frequency faster than the first horizontal scanning frequency is identified and selects the third operating mode when a third horizontal scanning frequency is identified faster than the second horizontal scanning frequency.
Yet another distortion correction system in accordance with inventive arrangements comprises: means for supplying different numbers of digital distortion correction values; a digital to analog converter for converting the supplied distortion correction values to an analog distortion correction signal; an analog low pass filter coupled for receiving the analog distortion correction signal and generating an analog deflection signal, the low pass filter being optimized only for those of the analog deflection signals having a given sample rate; means for determining a horizontal scanning frequency of an input video signal; and, control means responsive to the determining means for varying the different numbers of the supplied distortion correction values to maintain the given sample rate of the analog deflection signal for different horizontal scanning rates.
A method for distortion correction in accordance with inventive arrangements comprises the steps of: storing digital distortion correction values; digitally filtering the digital distortion correction values in a first mode of operation to generate interpolated distortion correction values; supplying as an output the distortion correction values together with the interpolated distortion correction values in the first mode of operation; supplying only the distortion correction values as the output in a second mode of operation; converting the supplied output to an analog distortion correction signal; low pass filtering the analog distortion correction signal in each of the operating modes with the same passive analog filter, for generating an analog deflection signal; determining a horizontal scanning frequency of an input video signal; and, responsive to the determining step, selecting one of the operating modes for different horizontal scanning frequencies.
A further method for distortion correction in accordance with inventive arrangements comprises the steps of: storing digital distortion correction values; digitally filtering the digital distortion correction values in a first mode of operation to generate a first number of interpolated distortion correction values; supplying as an output the distortion correction values together with the first number of interpolated distortion correction values in the first mode of operation; digitally filtering the digital distortion correction values in a second mode of operation to generate a second number of interpolated distortion correction values; supplying as the output the distortion correction values together with the second number of interpolated distortion correction values in the second mode of operation; converting the supplied output to an analog distortion correction signal; low pass filtering the analog distortion correction signal in each of the operating modes with the same passive analog filter for generating an analog deflection signal; determining a horizontal scanning frequency of an input video signal; and, responsive to the determining step, selecting one of the operating modes for different horizontal scanning frequencies.
The method can advantageously comprise the step of supplying only the distortion correction values as the output in a third operating mode. In this case, the method can advantageously comprise the step of selecting the first operating mode when a first horizontal scanning frequency is identified, selecting the second operating mode when a second horizontal scanning frequency is identified faster than the first horizontal scanning frequency and selecting the third operating mode when a third horizontal scanning frequency is identified faster than the second horizontal scanning frequency.
Yet another method for distortion correction comprises the steps of: supplying different numbers of digital distortion correction values; converting the supplied digital distortion correction values to an analog distortion correction signal; analog low pass filtering the analog distortion correction signal with a characteristic response optimized only for those of the analog deflection signals having a given sample rate; determining a horizontal scanning frequency of an input video signal; and, varying the different numbers of the supplied distortion correction values to maintain the given sample rate of the analog deflection signal for different horizontal scanning rates.
The inventive arrangements advantageously enable the same analog low pass filter, in particular a passive filter, to be used in a distortion correction system for a plurality of horizontal scanning frequencies including, for example, 1 fH, 2 fH and 3 fH. The use of a switchable, programmable digital filter, for example a finite impulse response filter with a selectable number of input taps, advantageously enables the nature and quantity of convergence correction values ultimately supplied to the low pass filter to be different for different horizontal scanning frequencies. The distortion values are advantageously controlled by selecting the number of interpolated values supplied with the original values as an output, and in one advantageous embodiment, by turning off the digital filter and supplying only original correction values. The switchable digital filter with selectable output values can be advantageously embodied in an integrated circuit, alone or bundled with other related functions.
The inventive arrangements take advantage of the realization that a critical factor in operation and optimization of the low pass filter resides in maintaining a constant or at least substantially constant sample rate in the signal being low pass filtered. The number of correction values supplied to generate the correction signal can be advantageously varied to maintain the same sample rate for different horizontal scanning frequencies. In particular, the number of interpolated values generated and supplied together with original values can be advantageously varied in accordance with inventive arrangements.